1. Field of the Invention
The present invention relates to disk drives. More particularly, the present invention relates to disk drives that are configured to compensate for spindle motor speed variations and disk eccentricity.
2. Description of the Prior Art
All disk drives, to varying extents, suffer from a condition in which the media are not mounted at the exact center of rotation of the spindle motor. This condition manifests itself as a certain degree of eccentricity as the disk is rotated by the spindle motor. Such eccentricity may be caused by a condition called “disk slip” in which the disk platters slip relative to the clamp and/or spacers mounting the media onto the spindle motor, which may occur as a result of a shock event occasioned by dropping or jarring the drive, for example. Alternatively, disk eccentricity may be caused, for example, by an inexact installation of pre-recorded media (from a media writer, for example) on the drive's spindle motor. Some eccentricity is unavoidable, as the center opening of the disk must, by definition, be larger than the spindle of the spindle motor onto which it must be fitted.
When a disk or disks rotate with eccentricity, the result is once per revolution (OPR) timing errors, also referred to as a “big run-out”. In contemporary drives, servo sectors of servo information are interspersed with data sectors circumferentially in concentric tracks around the recording surface or surfaces of the disk or disks. The format of the tracks calls for regularly spaced embedded servo sectors (also called servo wedges) containing servo information therein. Between the embedded servo sectors are a number of data sectors, which are configured to store user addressable data. When a disk rotates with eccentricity, the wedge-to-wedge timing will be modulated by the OPR timing error in a sinusoidal fashion as the disk rotates about the spindle, with the timing error being worse toward the inner diameter (ID) of the disk. This variation in the wedge-to-wedge timing due to disk eccentricity causes a certain degree of uncertainty in the timing of the servo control signals and the placement of user data. This timing uncertainty must be accounted for in the allocation of the various fields and the gaps interspersed between the data fields, which results in a loss of format efficiency, as compared to an ideal situation in which such timing uncertainty were minimized. For a given drive having 95,000 Tracks Per Inch (TPI) for example, a disk slip equivalent to a distance of 300 tracks in a 2.5 inch disk drive results in a 0.6% linear velocity uncertainty and a not insignificant 8% loss of user accessible storage capacity.
From the foregoing, it may be appreciated that methods for reducing the effects of disk eccentricity on the user addressable data spaces of the disk are needed.